Fill a large bowl three quarters full of water; add a handful of ice cubes. Cut onion into thin slices. Place in a strainer and lower into the water. Let stand for 20 minutes.

Preheat oven to 400 degrees.

Halve and core each pear; cut each half into six slices. Drain the onion wedges well and place them in a 9 by 13 inch glass baking dish along with the pear slices, oil, thyme, salt and a grinding of pepper. Toss to combine. Cover with foil.

Roast for 30 minutes, stirring twice.

Combine breadcrumbs and cheese in a small bowl. Drizzle with the remaining 2 T. of oil; stir to combine. Remove the pan from the oven, sprinkle the crumb mixture evenly over the gratin, return to the oven and roast until the breadcrumbs and are well browned 10  20 minutes. Let cool for 10 minutes before serving.